This invention relates generally to methods and apparatus for manufacturing semiconductor wafers and cutting wire apparatus for use in such methods and apparatus.
The particular properties of hard, brittle non-metallic semiconductor material having a Vickers hardness of up to HV 15,000 N/mm.sup.2 result in rigorous demands being placed on the cutting or slicing process by which wafers are manufactured from the material.
In the manufacture of semiconductor wafers, elongated bars or ingots are cast from or pulled out from a melt of the semiconductor material. Further processing of the material requires a slice-by-slice separation of these ingots into wafers. The separation process is characterized by at least the following two requirements:
(a) cutting losses must be minimized since the material is very expensive due to the complex manner in which it is obtained in highly pure form, e.g., the width of the cut should be significantly less than the thickness of the wafer, i.e., about 1 mm; and
(b) the sliced wafers should have surfaces which are as planar and as parallel to each other as possible.
Regarding the manner of cutting or slicing, arrangements in which looped cables are driven to cut through material have been known in principle for many years. For example, such arrangements have been used to cut rocks into square blocks. Generally, the moving cable is pulled against the workpiece while an abrasive agent and coolant-lubricant is applied whereby the cable slices through the workpiece. Although the cutting capacity of such arrangements is relatively modest, they are still in use today in quarries and in other applications in which precise cutting tolerances are not required.
Significant improvements in productivity of looped cable cutting processes has been achieved by providing abrasive agent directly onto the cable either in the form of discrete elements, such as by clamping a sleeve having an abrasive outer surface firmly onto the cable, or by fixing the abrasive agent onto the cable itself.
Since the precision of the cut obtained by a cutting cable as well as the cutting capacity of the cable both increase with increasing cable tension, cables having higher tensile strength are now used in many cable cutting applications. Indeed, traditional hemp rope no longer plays a role in modern industrial practice.
The present state of technology is such that it is quite possible to cut or slice very hard materials by means of cutting wire arrangements in which diamond coated wires or wires which function to carry loosely added cutting medium are used. A serious problem, however, arises from the demand for a minimum cutting width, such as in the case of manufacturing semiconductor wafers. In particular, whereas wires having a diameter as small as 1 mm enable the requisite tensile and cutting forces to be transferred to the wire using conventional drive mechanisms, the cutting width required in slicing wafers from ingots of semiconductor material necessitates reducing the wire diameter to only a few tenths of a millimeter. Such extremely thin cutting wires can only be used, however, if the tensile strength of the wire is approached in receiving the tensile and cutting forces. In this respect, the design of the arrangement by which the driving forces are applied to the cutting wire assumes special significance, i.e., the drive arrangement must be designed so that the tension in the wire, which is limited by the strength and thickness of the wire, will generate a maximum cutting force.
Countervailing considerations exist in connection with the tension and no-load forces acting on a cutting wire during a slicing operation. On the one hand, the tension and no-load forces must be sufficient to effect sliding of the wire within the cut being formed in the workpiece in order to complete a separation process. On the other hand, however, the same tension and no-load forces must not effect sliding of the wire over any of the drive components since this would cause the wire to cut the drive component itself, especially in the case where extremely thin cutting wires are used. This problem is magnified due to the widely fluctuating magnitude of the friction between the cutting wire and the workpiece and/or between the cutting wire and the drive mechanism.
The requirements described above are simultaneously achieved by providing that the extent to which the cutting wire wraps around sectors of the drive rollers of the drive mechanism on the driving side of the wire is sufficiently large. A large wrap-around curvature is generally obtained by providing several drive rollers over sectors of which the cutting wire is wrapped. However, the provision of several drive rollers in order to distribute forces over their respective sectors is useful on a practical basis only if the driving effect of the individual rollers are precisely adjusted with respect to each other. A proportionate division of torque resulting from an arrangement in which a common motor drives all of the drive rollers is not satisfactory since expansion slip results in unavoidable wear, particularly in the partial load region and primarily on the first drive roller. Such wear increases the maintenance intervals and, therefore, the down time of the machine. Moreover, a mechanical coupling of the drive rollers with each other is not practical using presently available technology. For example, a mechanical coupling of the drive rollers, such as by a toothed gear arrangement, limits the speed of rotation of the drive rollers, due to lubrication and other considerations, to speeds well below those required for wire cutting processes. The division of torque between the drive rollers obtained by friction couplings is not satisfactory since slippage between the drive rollers is unavoidable. Such slippage will of course be transmitted to the drive rollers wrapped by the cutting wire thereby resulting in increased wear of one or more of the drive rollers.
To the present, no solution has been found to the problems discussed above, and for these reasons looped cutting wire arrangements for use in applications where large cutting forces, minimum cutting widths and high precision are critical have not progressed beyond the experimental stage and have not been practical in industrial processes.